Cholesterol is vital to a variety of life-sustaining functions, such as serving as a source of fuel, contributing to cell structure, and manufacturing of hormones. For cholesterol to circulate through the blood, cholesterol typically combines with lipoproteins by esterification.
For instance, cholesterol can combine with low density lipoprotein (LDL) to form low density lipoprotein-cholesterol. LDL-cholesterol is a large spherical particle containing a core, which contains about 1,500 molecules of cholesterol. The core of cholesterol esters is enclosed in a layer of phospholipid and unesterified cholesterol molecules. The phospholipids are arrayed so that their hydrophilic heads are on the outside, thus allowing LDL-cholesterol to circulate through the blood.
However, too much cholesterol or LDL-cholesterol in the bloodstream is typically a major risk factor for cardiovascular disease. For example, excessive cholesterol can lead to formation of atheroscleromatous plaques. These plaques can cause narrowing and hardening of the arteries (i.e., atherosclerosis), which can impede blood flow and lead to a heart attack or stroke. In addition, relatively small atheroscleromatous plaques can become destabilized due to, for example, degradation of the connective tissue (i.e., collagen) “cap.” Destabilization of the plaques can result in rupture of the plaque and thrombosis, which can lead to myocardial infarction.
Another lipoprotein that can combine with cholesterol is high density lipoprotein (HDL). HDL typically transports cholesterol to the liver. The liver metabolizes cholesterol, thus removing it from the body. Therefore, HDL is beneficial since it aids the removal of excess cholesterol from the circulation.
Another risk factor that has been associated with cardiovascular disease is C-reactive protein (CRP), which can be measured in serum or plasma. CRP is released by the body in response to acute injury, infection or other inflammation-inducing conditions, such as atherosclerosis. The release of CRP in response to inflammation has been proposed as a potential biomarker for cardiovascular diseases, due to, for example, atherosclerosis. Accordingly, current research is focusing on developing drugs that inhibit CRP, and thus decrease the incidence of such diseases (Taubes, 2002. Science 296:242-245). For example, recent studies have shown that treatment with pravastatin (statin) appears to result in reduced levels of CRP (Ridker et al. 1999. Circulation 100:230-235).
Accordingly, elevated levels of LDL-cholesterol or CRP are serious predictors of cardiovascular disease. Elevated levels of both LDL-cholesterol and CRP are synergetic predictors of cardiovascular disease. Therefore, patients with both elevated levels of LDL-cholesterol and CRP have an increased risk for developing cardiovascular disease.
Current treatments for lowering cholesterol, LDL-cholesterol, or CRP include a class of drugs known as statins. Statins generally alter the metabolism of various constituents within the cholesterol metabolic pathway. These drugs typically reduce serum/plasma LDL-cholesterol levels and CRP levels.
However, statins are associated with numerous side effects, including elevation of plasma triglycerides, increased liver aminotransferase activity, abdominal discomfort, nausea, vomiting, diarrhea, malaise, QT interval prolongation, and decreased high-density lipoprotein levels. These side effects limit the effectiveness of statins.
The compound tetracycline is a member of a class of antibiotic compounds that is referred to as the tetracyclines, tetracycline compounds, tetracycline derivatives and the like. The compound tetracycline exhibits the following general structure: 
The numbering system of the tetracycline ring nucleus is as follows: 
Tetracycline, as well as the terramycin and aureomycin derivatives, exist in nature, and are well known antibiotics. Natural tetracyclines may be modified without losing their antibiotic properties, although certain elements must be retained. The modifications that may and may not be made to the basic tetracycline structure have been reviewed by Mitscher in The Chemistry of Tetracyclines, Chapter 6, Marcel Dekker, Publishers, New York (1978). According to Mitscher, the substituents at positions 5-9 of the tetracycline ring system may be modified without the complete loss of antibiotic properties.
In addition to their antibacterial properties, tetracyclines have been described as having a number of other uses. For example, tetracyclines are also known to inhibit the activity of collagen destructive enzymes, produced by mammalian (including human) cells and tissues, by non-antibiotic mechanisms. Such enzymes include the matrix metalloproteinases (MMPs), including collagenases (MMP-1, MMP-8 and MMP-13), gelatinases (MMP-2 and MMP-9), and others (e.g. MMP-12, MMP-14). See Golub et al., J Periodont. Res. 20:12-23 (1985); Golub et al. Crit. Revs. Oral Biol. Med. 2:297-322 (1991); U.S. Pat. Nos. 4,666,897; 4,704,383; 4,935,411; 4,9354,412. Also, tetracyclines have been known to inhibit wasting and protein degradation in mammalian skeletal muscle, U.S. Pat. No. 5,045,538; to inhibit inducible NO synthase, U.S. Pat. Nos. 6,043,231 and 5,523,297; to inhibit phospholipase A2, U.S. Pat. Nos. 5,789,395 and 5,919,775; and to enhance IL-10 production in mammalian cells. Tetracyclines have also been found to be useful for reducing CRP levels, U.S. application Ser. No. 60/395,466. These properties cause the tetracyclines to be useful in treating a number of diseases.
Several prior art references disclose the effect of tetracyclines on serum cholesterol levels. Some of these references report that tetracyclines had no effect on serum cholesterol. For example, Korpela et al. (Scand. J. Gastroenterol. 1984, 19:401-404) reported that administration of oxytetracycline had no effect on serum cholesterol and LDL-cholesterol in humans. Similarly, Samuel et al (Circ. Res. 1973. 33:393-402) disclosed that tetracycline had no effect on serum cholesterol levels in humans. Also, Berchev et al. showed that chlortetracycline either has no effect or increases serum cholesterol levels in cholesterol-fed rabbits.
In contrast, Bocker et al. (Arzneimittelforschung 1981. 31:211.8-2120), Shaddad et al. (Comp. Biochem. Physiol. 1985. 80:375-380), and Pigatto et al. Dermatologica. 1986. 172:154-159) reported that tetracyclines (e.g., tetracycline, doxycycline, oxytetracycline, and minocycline) reduce serum cholesterol levels. Similarly, Samuel et al. (Circ. Res. 1973. 33:393-402) reported that chlortetracycline reduces serum cholesterol levels and also reduces levels in patients with hypercholesterolemia.
Therefore, the prior art references are not consistent regarding the effect of tetracyclines on serum cholesterol levels. Furthermore, these references disclose studies on the effect of cholesterol levels by administering antibacterial tetracyclines. However, a disadvantage of using antibacterial tetracyclines is the development of antibiotic resistance to the tetracyclines and to other antibiotics (e.g., pan-antibiotic resistance). The possibility of using non-antibacterial tetracyclines was not disclosed in the prior art.
Thus, it is one object of the present invention to provide a method for decreasing elevated serum/plasma LDL-cholesterol levels. It would be especially desirable to provide a method for decreasing elevated serum/plasma LDL-cholesterol levels and C-reactive protein levels. Decreasing LDL-cholesterol levels and C-reactive protein levels would be surprising since it has been reported in the prior art that there is a lack of correlation between cholesterol levels and CRP levels in patients with cardiovascular diseases (Ridker, P et al. New England Journal of Medicine 2002. 347:1557-1565; Albert et al. JAMA 2001. 286:64-70; and Golub et al. unpublished data, see Example 2).